Natyashastra
हस्तौ तु सर्पशिरसौ मध्यमाङ्गुष्ठकौ यदा । तिर्यक्प्रसारितास्यौ च तदा सूचीमुखौ स्मृतौ ॥ १९२॥
When the two hands are in the shape of a serpent's head, with the middle finger and thumb touching, and are stretched obliquely, they are known as Sūcīmukha hands.
english translation
hastau tu sarpazirasau madhyamAGguSThakau yadA । tiryakprasAritAsyau ca tadA sUcImukhau smRtau ॥ 192॥
hk transliteration by Sanscript